The best way to value 3-4 unit buildings is using the sales comparison approach with nearby sales comps with similar attributes to the property.

In order of importance I do, Style of home, square footage, bed/bath count, basement comparison, then I move on to things like lot size, on or off street parking, garage, etc. 

Miami is a no-go—at least for the moment—as asking prices are exaggeratedly high, especially in comparison to assessed values.  

And also including area comps for comparison.

I look at costs based on flipping a house, so I look at doing better than the minimum.Has the seller provided any kind of breakdown for his sub $40k renovation.As a comparison, I'm under contract on a house with a $44,000 Reno budget. 1150 SF, with about 400 finished in the basement, 3/1.5.

Acquisition was ~30,000 and they're looking for ARV ~155k.After walking through this property we advanced to completed rehab property on concrete slab, tri-level for comparison work (it was actually just one block away from the fix/flip project.Aaron and Sean did a walkthrough, pointing out the finished work and having discussions about the costs of rehabs and financing options for doing fix/flips.Chicago Action Investors meet every weekend to form partnerships, gain knowledge from experienced investors, and analyze prospective properties for deals.

A blanket comparison of markets (or formula) will not be equally applicable across the board. 

For example, below is a comparison between the statewide energy cost in California vs.

The guidelines allow for this based on either current lease agreements, or, for vacant units, you can request as part of your appraisal a 1007 rent comparison schedule where the appraiser, in addition to determining the value of the property, will complete a 1007 form which basically determines what the vacant unit(s) would rent for based on market comparables. 

@Ross Greene, As I was reading through I was ready to argue against you again, but the lawyer comparison really put it into perspective for me!